1. Field of the Invention
The present invention relates to a semiconductor device and a fabrication method therefor, or in particular to a semiconductor device having a capacitor comprising a dielectric film made of a material having a high dielectric material, such as a dynamic random access memory, and a fabrication method therefor.
2. Description of the Prior Art
Electric circuits formed in semiconductor devices such as a dynamic random access memory (RAM) include various thin film capacitors. A dynamic random access memory needs a thin film transistor for each memory cell in order to hold signal charges in a very small area. In order to increase the degree of integration of a dynamic random access memory, it is required to fabricate a thin film capacitor in an area as small as possible. That is, it is needed to increase a capacitance of a thin film capacitor. The capacity of a thin film capacitor is proportional to a dielectric constant of a dielectric film. Then, in a semiconductor device described in Japanese Patent laid open Publication No. 44019/1991, a dielectric film of the capacitor is made of a material such as BaTiO.sub.3 having a high relative dielectric constant. In order to increase the degree of integration of a dynamic random access memory, it is needed to adopt a multi-layer structure with insulating films provided between circuits in layers.
However, a prior art thin film capacitor has a problem that the dielectric constant of the capacitor decreases by a leak current caused by defects in the dielectric film. A leak current is needed to be low in order to have a high capacitance enough to secure sufficient signal charges at a low voltage and to prevent dispersion of the charges. In general, there is a tendency that the capacitance can be increased by decreasing the thickness of the dielectric film while this increases the leak current due to trap levels in the film. In order to decease trap levels in order to decrease the leak current, annealing is performed on the material such as BaTiO.sup.3 having a high dielectric constant. However, in order to decease trap levels in order to decrease the leak current, annealing has to be performed at a temperature of 600.degree. C. or higher, and this high temperature process is liable to cause a bad influence to a transistor or the like fabricated below the thin film transistor. Further, even when such annealing is performed, the leak current is not sufficiently low, and it is desired to further decrease the leak current and to increase the dielectric constant.
Further, when a thin film capacitor is formed in a semiconductor device such as a DRAM, very fine patterns of a thin film are fabricated by etching with very fine resist patterns with ultraviolet rays of a short wavelength. As the degree of integration becomes larger as in a DRAM of 1 gigabits, it is expected that a minimum of processing width becomes 0.2 .mu.m or less. Then, exposure with soft X rays will be required. However, because X rays have an energy per photon, there are problems that defects are generated due to radiations in the dielectric film of the capacitor in a memory cell so that this deteriorates the characteristics of the thin film capacitor. Especially in a DRAM, metallization is performed after forming thin film capacitors. However, aluminum used in the metallization has a low melting point, it is difficult to recover the characteristics of the thin film capacitors by annealing or the like.
Platinum is usually used for a lower electrode of a thin film capacitor. However, this has a problem because a material below the lower electrode disperses through the lower electrode to the dielectric film, to decrease a dielectric constant of the thin film capacitor.
Especially, in a multi-layer DRAM for high integration, a lower electrode of a thin film capacitor is connected through a silicon plug to a transistor formed below the capacitor. However, in a high temperature process (&gt;350.degree. C.), silicon disperses through the plug to the dielectric film, and this also decreases a dielectric constant of the thin film capacitor.
When platinum is used for a lower electrode of a thin film capacitor, it is difficult to form a reaction layer between an interface with a dielectric film. However, an amorphous layer having a low dielectric constant is liable to be formed at an interface with the dielectric film due to lattice mismatching between the platinum electrode and the dielectric film.